The present invention relates to a method of diagnosing changes in steering force with respect to changes in vehicle velocity.
A generally used power steering system will be described first with reference to FIG. 2. Referring to FIG. 2, reference numeral 1 denotes an engine; 2, a vehicle velocity sensor; 3, an oil pump; 4, a tank; 5, outlet-side piping; 6, return-side piping; 7, a controller; 8, a current feeder line; 9, an ignition key; 10, a battery; 11, a solenoid valve; 12, a control valve of power steering; 13, a steering shaft; 17, an indicator lamp; and 18, power source backup wiring. Reference symbol SW1 denotes a diagnosis switch; and SW2, a steering force switch.
An operation of the power steering system will be described below. A normal operation will be described first. When the ignition key 9 is turned on to start the engine 1, the oil pump 3 is rotated by the engine 1 so as to supply an oil to the control valve 12 of power steering and the like as a pressurized oil. The supplied oil passes through the control valve 12, the solenoid valve 11, and the like and returns to the tank 4 through the return-side piping 6. The engine 1 and the vehicle velocity sensor 2 respectively output an engine speed signal a and a vehicle velocity signal b to the controller 7. Upon reception of the engine speed signal a and the vehicle velocity signal b, the controller 7 supplies a current corresponding to the signals a and b to a solenoid 14 so as to control a reactive pressure, thus generating a reactive steering force corresponding to a vehicle velocity.
FIGS. 3(a) and 3(b) show the solenoid valve 11 in detail. Referring to FIG. 3, reference numeral 14 denotes the solenoid connected to the current feeder line 8; 15, a spool to be driven by the solenoid in the lateral direction in FIG. 3; and 16, a reactive steering force chamber in the control valve 12 of power steering. FIG. 3(a) shows a state of the solenoid valve 11 during high-velocity travel of the vehicle. In this state, the spool 15 is in an "open" state, and a predetermined pressure is applied from the oil pump 3 to the reactive steering force chamber 16. This predetermined pressure is converted into a torque to generate a predetermined reactive steering force. FIG. 3(b) shows a state of the solenoid valve 11 during stationary turn (low-velocity travel) of the vehicle. In this state, the spool 15 is in a "closed" state, and the pressure of the reactive steering force chamber 16 is set to be equal to that of the tank 4. Hence, the reactive steering force becomes zero.
In delivery inspection, the above-described operation of the power steering system must be checked. FIG. 4 shows a conventional diagnosis sequence for this purpose. The ignition key 9 and the diagnosis switch SW1 (see FIG. 2) are turned on first (step 21). With this operation, the vehicle is kept in a self-diagnosis mode until the ignition key 9 is turned off. This diagnosis switch SW1 is arranged at a position where a user cannot operate and is normally kept OFF. While the diagnosis switch SW1 is OFF, the vehicle is set in a normal travel mode instead of the diagnosis mode indicated by the flow chart in FIG. 4.
The engine is started by a starter switch to perform an idling operation (step 22). Since the vehicle is stopped at this time, the solenoid 14 is driven by a current (about 0.95 A) corresponding to a vehicle velocity of 0 km/h. Subsequently, the solenoid 14 is kept driven by this current.
It is checked whether a vehicle velocity signal is proper, i.e., pulses are input from the vehicle velocity sensor 2 (step 24). If the signal is proper, a flag representing that the vehicle velocity signal is proper is set in an internal memory (step 25). Since the vehicle is not travelling immediately after the self-diagnosis mode is set, no pulses are input and hence no proper vehicle velocity signal is normally obtained. However, as will be described later, the sequence is repeatedly executed as indicated by step 35. Therefore, if the flow advances to step 24 after the vehicle starts to travel, since a predetermined number of pulses (e.g., seven pulses) or more pulses are held in a counter for counting a vehicle velocity signal, it is determined that the vehicle velocity signal is proper. If no pulses are counted after the vehicle starts to travel, it means failure of the vehicle velocity sensor 2, disconnection of the wiring through which the vehicle velocity signal b flows, or the like. In this case, no flag indicating the vehicle velocity signal is proper is set until self-diagnosis is completed. During a self-diagnosis operation, the vehicle is caused to travel only several meters. For this reason, the vehicle velocity is limited to about 10 to 20 km/h.
Subsequently, it is checked whether an engine speed signal is proper (e.g., 21 pulses or more) (step 26). If the signal is proper, a flag representing that the engine speed signal is proper is set (step 27). If the wiring through which the engine speed signal a flows is disconnected or the like, no flag is set. It is then checked whether the solenoid is properly operated (step 28). If YES in step 28, a flag representing that the solenoid is properly operated is set (step 29). If NO in step 28, current supply to the solenoid 14 is stopped (step 30). Whether the solenoid 14 is properly operated or not is determined by checking whether the driving current value set in step 23 is a preset value. While the vehicle is not travelling, for example, about 0.95 A is set as the preset value, A driving current may become larger than the preset value due to short circuit of the solenoid 14 or the like. In this case, current supply to the solenoid 14 must be immediately stopped.
It is checked whether a backup power source is proper (step 31). If YES in step 31, a flag representing that the backup power source is proper is set (step 32). This backup power source is supplied from the battery 10 through the wiring 18 in FIG. 2 and is used to back up a RAM (not shown) for storing the respective data during vehicle travel. It is then checked whether the steering force switch SW2 is properly operated (step 33). If YES in step 33, a flag representing that the switch SW2 is properly operated is set (step 34). It is determined that the switch SW2 is properly operated if, for example, one of sport and normal modes is selected once or more. The switch SW2 serves to select either a large steering force or a normal steering force. When the normal mode is selected, a normal steering force is required. When the sport mode is selected, a large steering force is required. Assume that the switch SW2 is set in the normal mode. In this case, if an operator switches the switch SW2 to the sport mode, this switched state is detected, and it is determined that the switch SW2 is properly operated.
It is checked whether the diagnosis switch SW1 is ON (step 35). The processing from step 24 to step 35 is repeated as long as the switch SW1 is ON.
After the diagnosis switch SW1 is turned off, the solenoid is stopped and a diagnosis result is output (steps 36 and 37). Whether the diagnosis result is proper or abnormal is determined by checking whether a flag is set in a corresponding internal memory. The display lamp 17 is turned on and off in accordance with the decision made on the diagnosis result or an object to be diagnosed.
In conventional diagnosis of the power steering system in the self-diagnosis mode, since a vehicle is caused to travel only several meters, the vehicle velocity cannot be increased much. For this reason, it is not possible to check changes in steering force with respect to changes in vehicle velocity in actual travel. Therefore, errors associated with changes in steering force cannot be detected.
In order to accurately diagnose changes in steering force with respect to changes in vehicle velocity, a vehicle must actually travel at considerably high speed. However, it is practically impossible to perform such an operation in delivery inspection.